Heretofore, representative methods of laminating a first substrate film layer and a second substrate film layer were extrusion lamination and dry lamination, which are widely utilized now. Extrusion lamination comprises applying an anchor coating (AC) agent, such as an imine-based or urethane-based one, dissolved in a solvent, to the first substrate film layer, followed by drying, and extruding to laminate the second substrate film layer thereon (Masayoshi Araki, “Lamination Processing Handbook”, pp 25-32, Kakogijutsu Kenkyu-Kai, 1978). Dry lamination comprises applying an adhesive, such as a urethane adhesive, dissolved in a solvent, to the first substrate film layer, followed by drying, and pressing to laminate the second substrate film layer thereto (ibid. pp 14-18).
On the other hand, as a lamination method without using a solvent, there are the non-solvent lamination of the polyol-isocyanate type and hot-melt lamination. The non-solvent lamination comprises applying a non-solvent reactive type adhesive resin, of which the viscosity has been lowered by heating, to the first substrate film layer, and pressing to laminate the second substrate film layer thereon (ibid. pp 48-49). The hot-melt lamination comprises melting a hot-melt adhesive resin, applying it to the first substrate film layer, and laminating the second substrate film layer thereon (ibid. pp 19-24).
However, in the above extrusion lamination and dry lamination, an organic solvent, such as ethyl acetate, toluene or isopropyl alcohol, is used in a quantity for dissolving the adhesive, and causes noxious odors on working, degradation of working atmosphere and risk of explosion. Furthermore, the organic solvent contaminates the environment around the factory which is forced to move to the suburbs, as well as regulations therefor become severe. The solvent is evaporated resulting in wasting resources. Moreover, unless the solvent is sufficiently evaporated, the odor of the solvent remains in the laminate. Particularly, in the case of using it for food packaging, it is a problem regarding food hygiene.
With respect to extrusion lamination, although the productivity is good due to a fast production speed, the laminated strength of the laminate is inferior to a laminate produced by dry lamination. As a result, it cannot be used as a packaging material for retort foods.
On the other hand, in the case of dry lamination, the laminated strength of the laminate is the strongest, and is used for most of the packaging materials for boiled foods and retort foods. However, the production speed is slow compared with extrusion lamination, and aging of the polyurethane adhesive at 40-70° C. in a temperature controlled room for 2-7 days is necessary for curing, which requires a large place for the temperature controlled room and great energy cost. Moreover, toluene diisocyanate (TD1) used as a curing agent has a risk of being converted into toluene diamine (TDA) which is said to be carcinogenic. In order to avoid this risk, when the curing agent is changed from TD1 to an aliphatic isocyanate having a slow curing rate, uncured polyurethane adhesive resin effuses into retort foods to cause a foreign taste and odor and to be unfavorable in view of food hygiene.
In the above non-solvent lamination and hot-melt lamination, the viscosity of the coating solution must be low for application, and such a restriction of the viscosity restricts the molecular weight of the adhesive resin to be low. As a result, the laminated strength of the laminate becomes much smaller than a laminate manufactured by dry lamination. Then, hot-melt lamination is frequently used for the manufacture of easily peelable materials.
Furthermore, in non-solvent lamination and hot-melt lamination, since the adhesive resin is applied in a molten state, troubles occur on applying, such as solidification.